Question

[tex]S1= \frac{1}{1*2}+ \frac{1}{2*3}+ \frac{1}{3*4}+...+ \frac{1}{19*20}
[/tex] cum fac adunarea asta? explicati

Answer 1

se foloseste formula 1/n(n+1)=1/n-1/n+1⇒S1=1-1/2+1/2-1/3+.........+1/19-1/20         ⇒S1=1-1/20=19/20